In its broadest sense, this invention comprehends a composite substrate structure finding utility in the fabrication of electronic devices. The composite structure includes a support member which is a generally rigid (i.e. capable of operably supporting not only itself, but electrical and semiconductor layers deposited thereupon) sheet having a relatively thin electrically insulating coating deposited thereupon, the coating having chemical, mechanical, morphological and structural surface properties making it well-suited for the subsequent deposition of semiconductor layers thereonto.
Electronic devices are generally fabricated upon substrates which are adapted to provide (1) structural support to subsequently deposited regions of thin film metallic, electronic or semiconductor layers and (2) electrical isolation between adjacent regions of those metallic, electronic or semiconductor layers. Since the deposited metallic or semiconductor material is routinely less than a micron in thickness, the support afforded by the substrate is required to supply rigidity, thereby enabling the devices to be handled during fabrication, installation and repair. And because adjacent circuit elements and regions of the devices formed by the metallic, electronic, or semiconductor regions may be horizontally separated from one another by distances as small as a micron, good electrical isolation between those circuit elements and regions is necessary to prevent information contained in the circuit from being comingled and rendered meaningless. Further, in electronic devices employing a metallic substrate, an electrically insulating layer is necessary to vertically separate the metallic and semiconductor material from said metallic substrate, i.e., prevent short circuiting current through the common substrate. Therefore, electrical insulation and isolation must be provided in both the vertical and the horizontal directions in order to form the many types of operative electronic devices it is possible to manufacture utilizing the technology disclosed herein.
As used hereinafter, the term "electronic device" refers to a discrete device in the form of a single electronic component or circuit element which may either be used as is, or interconnected to other electronic components or circuit elements. A discrete device may comprise a single circuit element such as a transistor, diode, triode, photocell, etc.; or, may comprise an integrated circuit including therein a plurality of such circuit elements. The electronic device may be electrically interconnected to other devices to form interactive arrays such as photovoltaic arrays, memory arrays and the like. All semiconductor devices are specifically included in the definition of electronic devices used herein.
The selection and use of an appropriate substrate is especially important in the fabrication of thin film electronic devices. Such devices are typically fabricated by the sequential deposition of thin, patterned layers of semiconductor material and/or electrically conductive material in configurations chosen to provide a preselected flow of current between and through those layers. Such thin film devices have several advantages over monolithic single crystal devices insofar as they can (1) be economically fabricated, (2) cover large surface areas, and (3) also be deposited in a continuous production process. Since the films deposited on the substrates may be substantially less than one micron thick, they are incapable of self-support, and therefore must rely upon a substrate for rigidity. Furthermore, and as previously mentioned, in many instances the substrate must be electrically insulating in both the vertical direction to prevent shorting between the substrate and the superimposed metallic, electronic or semiconductor layers, and the horizontal direction to prevent shorting between adjacent electrically conductive circuits or circuit elements deposited upon the substrate.
Large area arrays of thin film electronic devices may be fabricated from a plurality of electrically isolated, discrete, circuit elements (diodes, triodes and transistors) which are deposited and interconnected upon the single large area substrate. In this manner large area logic arrays, memory storage devices, photovoltaic generators, display devices, etc. may be continuously and economically fabricated.
As previously described, when the electronic device includes a plurality of amorphous semiconductor layers, each individually deposited layer may be no more than a couple of hundred angstroms thick. As a result the layers, even when taken in toto are quite sensitive to defects and irregularities in the morphology of the substrate. More specifically, a crater descending onto the surface of the substrate or a projection arising from the surface of the substrate may not be fully covered or filled by the subsequently deposited thin layers of semiconductor material. If these morphological deviations from a smooth substrate surface are not fully covered by each layer of semiconductor material, current shunting paths are formed, thereby decreasing, if not totally destroying, the efficiency of the electronic device produced therefrom. Accordingly, the morphological quality of the substrate surface is an essential property of the substrate, the tolerances of which must be duly respected.
For reasons of economy, the large scale, continuous mass production of relatively large area, thin film electronic and semiconductor devices is of great commercial importance. The use of a thin, continuous, flexible substrate represents a means of economizing previous continuous processing techniques. While continuous processing would be possible utilizing non-flexible, non-continuous substrates, cost efficiency favors the continuous deposition of thin film electronic, metallic and semiconductor layers upon a continuous, flexible substrate in a roll-to-roll process. In this manner the production of amorphous electronic and semiconductor devices can complete with their crystalline counterparts in a cost effective manner.
Accordingly, it is one object of the instant invention to provide a thin, continuous, flexible substrate for electronic devices, said substrate (1) having sufficient rigidity to support the thin film layers of electronic, metallic and semiconductor material from which the device is fabricated during processing, handling, installation and repair thereof and (2) being electrically insulating in both the vertical and horizontal directions so as to provide electrical isolation both between the substrate and the superimposed layers of semiconductor material (vertical isolation), and between adjacent circuits and circuit elements formed upon the substrate (horizontal isolation). It is a further object of the instant invention to provide a thin, continuous substrate having sufficient flexibility to permit the continuous roll-to-roll fabrication of electronic devices thereupon. It is still another object of the instant invention to provide a thin, continuous, flexible substrate adapted to be used for the fabrication of electronic devices, the substrate having sufficient morphological smoothness and regularity to prevent current shunting defects from short circuiting electronic devices, produced thereon.
Previous substrate materials utilized for the production of electronic and semiconductor devices included rigid sheets of electronically insulating material, such as alumina, beryllia, glass, and quartz. While these substrate materials provided (1) good electrical isolation and mechanical support for electronic devices formed thereon, and (2) a morphologically smooth surface upon which to deposit semiconductor material, these substrates possess other inherent characteristics which render the fabrication of electronic and semiconductor devices therefrom cumbersome, if not impossible. For instance, glass and ceramic materials are relatively heavy, brittle and expensive. Further, processing steps involving such heavy, brittle substrates can become fairly complex due to problems associated with the handling and storage thereof. Additionally, electronic and semiconductor devices formed upon such substrates are relatively bulky and very heavy, thereby further reducing the weight and flexibility advantages which could otherwise be realized from the utilization of thin metallic and semiconductor films.
Plastic substrates are well known and have been utilized for sometime in the electronics industry for fabricating printed circuit boards and the like. However, plastic substrates have not heretofore been advantageously employed for the formation of thin film semiconductor layers directly thereupon. This is believed to be due to the facts that plastic substrates (1) do not provide sufficient morphological quality to deposit semiconductor layers without developing current shunting defects, (2) must be relatively thick in order to provide sufficient rigidity to accept stresses which build up in the deposited semiconductor layers, and (3) if fabricated of sufficient thickness and surface quality, would be too expensive to economically compete with other substrate materials.
Thin, metallic substrates offer several advantages over their plastic counterparts insofar as they (1) are rigid yet flexible, (2) may be made relatively thin without losing rigidity, (3) may be provided with a smooth surface finish, and (4) are relatively inexpensive. However, such thin metallic substrates neither provide (1) horizontal electrical isolation when fabricating electronic devices from contiguous circuits and circuit elements, nor (2) vertical electrical isolation when fabricating electronic devices from successive layers of metallic, electronic or semiconductor material deposited successively onto the substrate. Various layers of insulating material have previously been applied atop metallic substrates in the production of semiconductor and electronic devices, however, such insulating layers generally were formed of relatively thick coatings of ceramic materials. Accordingly, such insulated metallic substrates were not sufficiently flexible and/or did not present a sufficiently smooth surface for the deposition of thin film metallic circuitry or semiconductor layers thereonto.
Composite substrates, fabricated according to the principles of the instant invention, offer sufficient structural rigidity and good electrical isolation, whereby deposition of metallic and electronic circuitry and semiconductor material directly thereupon is possible. Furthermore, such composite substrates may be made sufficiently flexible so as to be employed in a roll-to-roll continuous process for the production of very large area electronic devices. Most importantly, composite substrates of the instant invention are characterized by a high quality surface finish which is optimized to substantially eliminate current shunting defects which would otherwise harm the efficiency of, if not totally destroy, thin film electronic devices produced by the deposition of the metallic, electronic or semiconductor material onto that surface.
There exists one additional capability which can be achieved through the use of the composite substrate of the present invention. In the manufacture of semiconductor devices such as photovoltaic cells, the efficiency of the devices may be increased by forming back reflectors on the surface of the substrate upon which the amorphous semiconductor materials are subsequently deposited. These back reflectors may be either specular or diffuse. With either type of reflector, light which has initially passed through the active region or regions of the devices, but which is unabsorbed or unused, is redirected from the back surface of the device by the reflector to pass, once again, through those active regions. The additional pass results in increased photon absorption and charge carrier generation in the active regions, thereby providing increased short circuit currents. In the case of specular back reflectors, the unused light is generally redirected for one additional pass through the active regions of the device. Contrarily, in the case of diffuse back reflectors, the light is scattered in addition to being redirected through the active regions, causing a portion of the redirected light to travel at angles sufficient to cause it to be substantially confined within the device by internal reflection, thereby producing multiple reflections of the redirected light through the active regions. As a result, both specular and diffuse back reflectors provide for increased short circuit currents and thus increased efficiencies. Another advantage of the diffuse reflector is that since the directed light passes through the active regions of the device at an angle, the active regions can be thinner to reduce charge carrier recombination, while still maintaining efficient charge carrier generation and collection.
Such specular and diffuse reflectors are described in detail in U.S. patent application Ser. No. 422,688 of Izu, et al, filed Sept. 24, 1982 now abandoned, and entitled "Apparatus and Method For Making Large Area Photovoltaic Devices Incorporating Back Reflectors; and U.S. patent application Ser. No. 359,371 of Cannella, et al, filed Mar. 18, 1982 and entitled "Improved Back Reflector System And Devices Utilizing Same". Both of the foregoing applications are assigned to the assignee of the instant invention and are incorporated herein by reference.
Unfortunately, problems existed with the methods employed, prior to the instant invention, to form either specular or diffuse back reflectors on the deposition surfaces of substrates. In the case of specular reflectors, it was difficult to deposit a mirror-like layer of a reflective material, such as gold, silver, aluminum, copper, chromium, or molybdenum, onto the substrate in a manner which would not subsequently peel. In the case of diffuse reflectors, a roughened texture was achieved by techniques such as sandblasting the substrate surface. However, such texturizing techniques resulted in the formation of morphological projections and craters of uncontrollable height and depth. As previously stressed, due to the very thin thickness of the deposited semiconductor layers, a projection extending upwardly from the surface of the substrate is likely not to be covered by the thin semiconductor layers. The result is a plurality of current shunting defects which render the photovoltaic device useless. Both the problems arising in the production of diffuse and specular back reflectors are solved by the use of the composite substrate disclosed in the present application. The morphologically smooth exterior of the insulating layer of the substrate provides a surface (1) upon which the metallic reflective material can be permanently applied for specular reflection, and (2) which can itself be controllably textured for diffuse reflection without simultaneously forming current shunting defects.
Recently, considerable efforts have been made to devleop systems for depositing amorphous semiconductor alloy materials, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type semiconductor devices which are, in operation, substantially equivalent to their crystalline counterparts. It is to be noted that the term "amorphous", as used herein, includes all materials or alloys which have long range disorder, although they may have short or intermediate range order or even contain, at times, crystalline inclusions.
It is now possible to prepare amorphous silicon and germanium alloys by glow discharge deposition or vacuum deposition techniques, said alloys possessing (1) acceptable concentractions of localized states in the energy gaps thereof, and (2) high quality electronic properties. Such techniques are fully described in U.S. Pat. No. 4,226,898, entitled "Amorphous Semiconductors Equivalent To Crystalline Semiconductors", issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Pat. No. 4,217,374, issued in the names of Stanford R. Ovshinsky and Mastasugu Izu on Aug. 12, 1980, also entitled "Amorphous Semiconductor Equivalent To Crystalline Semiconductors"; and U.S. patent application Ser. No. 423,424 of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens entitled "Method of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy". As disclosed in these patents and application, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498, issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (VOC). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the currents generated in each cell, the overall open circuit voltage becomes the sum of the open circuit voltage generated in each cell, while the short circuit current thereof remains substantially constant.
Unlike crystalline materials which are limited to batch processing for the manufacture of solar cells, amorphous semiconductor alloys can be deposited in multiple layers over large area substrates to form semiconductor devices in a high volume, continuous processing system. Such continuous processing systems are disclosed in pending patent applications and patents: Ser. No. 151,301, filed May 19, 1980, for "A Method Of Making P-Doped Silicon Films And Devices Made Therefrom" now U.S. Pat. No. 4,400,409; Ser. No. 244,386, filed Mar. 16, 1981, for "Continuous Systems For Depositing Amorphous Semiconductor Material"; Ser. No. 240,493, filed Mar. 16, 1981, for "Continuous Amorphous Solar Cell Production System" now U.S. Pat. No. 4,410,588; Ser. No. 306,146, filed Sept. 28, 1981, for "Multiple Chamber Deposition And Isolation System And Method" now U.S. Pat. No. 4,438,723; Ser. No. 359,825, filed Mar. 19, 1982 for "Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells" now U.S. Pat. No. 4,492,181; and Ser. No. 460,629 filed Jan. 24, 1983 for "Method And Apparatus For Continously Producing Tandem Amorphous Photovoltaic Cells" now U.S. Pat. No. 4,485,125. As disclosed in these patents and applicat1ons, a substrate may be continously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device of p-i-n type configuration, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy.
The layers of semiconductor material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form a photovoltaic device including one or more p-i-n cells, one or more n-i-p cells, a Schottky barrier, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
In addition to being utilized for the fabrication of photovoltaic devices and similar large area photovoltaic arrays, the foregoing techniques of depositing thin film amorphous semiconductor materials can be utilized to form electronic devices utilizing or incorporating one or more triodes, transistors, diodes and combinations thereof. In this manner large area (one foot square or greater) electronic devices such as memory devices, signal processing devices, display devices and the like may be prepared. The instant invention therefore provides a composite substrate layer having great utility in devices of this type.
These and other advantages of the instant invention will become apparent from the drawings, the detailed description of the invention and the claims which follow.